Can I let a tracking marker paint a mask?
You will probably need to do it with dynamic paint, as Charles recommended, and it is possible without solving a camera motion:

You can easily set it up by tracking the wall where he is painting with a simple 4 point track, so you get 5 markers. Then, link empties to those markers, hook a plane's vertices to the wall markers and parent a cube or other object to the "painting" marker, so you can get a canvas (plane) that deforms with the footage without solving or reconstructing geometry.

Set the dynamic paint to output an image sequence of the object painting's on the plane, and then use that image sequence in the compositor as the mask, or alpha, that you need to create the effect that he is painting it.